Compact lighting system and display device

ABSTRACT

A compact backlight system has a light-emitting panel ( 1 ) with a front wall ( 2 ), a rear wall ( 3 ) situated opposite thereto, and furthermore, between the front and the rear wall ( 2, 3 ), a translucent input edge surface ( 4 ) for coupling light into the light-emitting panel ( 1 ). A light source ( 6, 6 ′, . . . ) is associated with the input edge surface ( 4 ). According to the invention, the rear wall ( 3 ) in a first portion ( 12 ) of the light-emitting panel ( 1 ) is provided with a multiplicity of steps ( 13, 13 ′, . . . ), and a second portion ( 22 ) of the light-emitting panel ( 1 ) widens from the input edge surface ( 4 ) in a direction facing the first portion ( 12 ). Preferably, a surface ( 17 ) of the steps ( 13, 13 ′, . . . ) facing the input edge surface ( 4 ) makes an angle β av ≧5° (with respect to a normal on a bisecting plane ( 20 ) bisecting the light-emitting panel ( 1 ).

The invention relates to a lighting system provided with alight-emitting panel comprising

-   -   a front wall, a rear wall situated opposite thereto, and        furthermore, between the front and the rear wall, a translucent        input edge surface for coupling light into the light-emitting        panel,    -   while at least a light source is associated with the input edge        surface, and    -   while, in operation, light originating from the light source is        incident on the input edge surface and distributes itself in the        light-emitting panel.

The invention also relates to a display device provided with saidlighting system.

Such lighting systems are known per se and are also referred to as edgelighting systems. They are used inter alia as backlighting systems in(picture) display devices, for example for TV sets and monitors. Suchlighting systems are particularly suitable for use as backlights fornon-emissive displays such as liquid crystal display devices, alsoreferred to as LCD panels, which are used in (portable) computers or(portable) telephones.

Said display devices usually comprise a substrate provided with aregular pattern of pixels which are each controlled by at least oneelectrode. The display device utilizes a control circuit for achieving apicture or a datagraphic display in a relevant field of a (picture)screen of the (picture) display device. The light originating from thebacklight in an LCD device is modulated by means of a switch ormodulator, various types of liquid crystal effects being used. Inaddition, the display may be based on electrophoretic orelectromechanical effects.

Such lighting systems are also used as luminaires for general lightingpurposes or for shop lighting, for example shop window lighting orlighting of (transparent or semi-transparent) plates of glass orsynthetic resin on which items, for example jewelry, are displayed. Suchlighting systems are further used as window panes, for example forcausing a glass wall to radiate light under certain conditions, or toreduce or block out the view through the window by means of light. Afurther alternative application is the use of such lighting systems forilluminating advertising boards.

In the lighting systems mentioned in the opening paragraph, the lightsource used is usually a tubular low-pressure mercury vapor dischargelamp, for example one or a plurality of cold-cathode fluorescent lamps(CCFL), wherein the light emitted by the light source during operationis coupled into the light-emitting panel, which acts as an opticalwaveguide. This waveguide usually constitutes a comparatively thin andplanar panel which is manufactured, for example, from synthetic resin orglass, and in which light is transported through the optical waveguideunder the influence of (total) internal reflection.

As an alternative light source, such a lighting system may also beprovided with a plurality of optoelectronic elements, also referred toas electro-optical elements, for example electroluminescent elements,for example light-emitting diodes (LEDs). These light sources areusually provided in the vicinity of or tangent to a light-transmittingedge surface of the light-emitting panel, in which case lightoriginating from the light source is incident on the light-transmittingedge surface during operation and distributes itself in the panel.

A lighting system is known from U.S. Pat. No. 5,575,549. Light from alinear light source with a uniform brightness is coupled into alight-emitting panel, also referred to as light pipe, via a lightincident edge surface. Part of the light transmitted through said lightincident edge surface is made incident upon inclined surfaces of conicalor polygonal pyramid-shaped so-called concave portions. Light is coupledout of the light-emitting panel after the light has been refracted atthe concave portions in the light-conducting member.

A drawback of the known lighting system is that the uniformitydistribution of the light coupled out of the light-emitting panel stillis relatively poor.

It is an object of the invention to eliminate the above disadvantagewholly or partly. According to the invention, a lighting system of thekind mentioned in the opening paragraph is for this purposecharacterized in that the rear wall in a first portion of thelight-emitting panel is provided with a multiplicity of steps, and inthat a second portion of the light-emitting panel widens from the inputedge surface in a direction facing the first portion.

Since the second portion of the light-emitting panel widens from theinput edge surface towards the first portion, the light coupled into thelight-emitting panel at the input edge surface cannot leave thelight-emitting panel during its first travel through the light-emittingpanel from the input edge surface towards the first portion. The angularlight distribution of light traveling through such a wedge-shaped secondportion of the light-emitting panel becomes gradually narrower.Generally speaking, light is collimated in a light-emitting panel whichwidens. In a light-emitting panel based on total internal reflection(TIR), light not exceeding a limit angle cannot be coupled out of alight-emitting panel. In the reverse situation, in which light travelsthrough a light-emitting panel which narrows, the angular lightdistribution becomes gradually greater. Since the light coupled into thelight-emitting panel at the input edge surface during its first travelthrough the second portion of the light-emitting panel cannot leave thissecond portion of the light-emitting panel, it is promoted that thelight during this travel through the second portion of thelight-emitting panel distributes itself in the light-emitting panel, andalso that the light, if originating from two or more, possiblydifferently colored light sources, is satisfactorily mixed. A gooddistribution and/or mixing of light promotes the uniformity of the lightissued from the light-emitting panel. The second portion of thelight-emitting panel acts as if it were a light-mixing chamber for lighton its first path through a widening second portion of thelight-emitting panel. Such a “light-mixing chamber” is usually providedoutside the light-emitting panel in known lighting systems, which is whysuch light-emitting panels occupy more space than necessary. Byemploying a light-emitting panel with a wedge-shaped second portionaccording to the invention, the “light-mixing chamber” is integratedinto the light-emitting panel leading to a considerable saving in space.

It is noted that wedge-shaped light-emitting panels are known per se(see, e.g. the lighting system known from U.S. Pat. No. 5,575,549), butsuch known wedge-shaped light-emitting panels are widest at the area ofthe edge surface where light is coupled into the light-emitting paneland narrowest at the opposite end of the light-emitting panel.

Normally, the second portion would end in a reflecting edge surface onthe side of the light-emitting panel facing the input edge surface.After reflection the angular light distribution increases again, independence on the manner in which the reflection takes place. On itsreturn path through such a light-emitting panel, light will have afurther increasing angular distribution owing to the fact that thelight-emitting panel gradually narrows. At a certain moment the angularlight distribution will have become so wide that part of it exceeds thelimit angle for total internal reflection, and light will issue from thelight-emitting panel. Normally, upon reflection the light is given atilt such as to promote the output of light on its way back in thedirection of the input edge surface. However, such a tilt given to thereflected light should be such that the light is coupled out over thewhole area of the front wall of the light-emitting panel to uniformlyilluminate the display device. Under said reflection conditions a brightband of light is issued from the front wall of the light-emitting panelin the vicinity of said reflecting edge surface. The brightness of saidband of light is substantially higher than the light emitted from therest of the front wall of the light-emitting panel. A solution would beto exclude this bright band of light from the light contributing toilluminating the display device. However such shielding is spaceconsuming and the system efficiency would be reduced.

The inventors have had the insight that the above-described bright bandof light originated from light coupled out directly after reflection atthe reflecting edge surface. The light coupled out at the bright band oflight is practically the sum of all light coupled in at solid angleslarger than a limit angle, for which the angle after reflection at thereflecting edge surface is equal to the critical angle for TIR.According to the measure of the invention the rear wall in the firstportion of the light-emitting panel is provided with a multiplicity ofsteps. As a consequence, the bright band of light is split into manyshorter bright bands of light which are spread over the front wall ofthe light-emitting panel. The spatial frequency of the steps in thefirst portion of the light-emitting panel can be chosen such that thebrightness modulation from the many small bands of light can no longerbe resolved, so that the brightness is averaged and appears homogeneous.The averaged, i.e. lower brightness of the bright band of light can betuned so as to be equally high as the brightness of the remaining partof the front wall.

A particularly compact lighting system is obtained through the measureaccording to the invention, with a high uniformity of the distributionof the light emitted by the lighting system. A more uniform illuminationof the display device is realized thereby, in particular in the case of(picture) display devices.

A preferred embodiment of the lighting system according to the inventionis characterized in that a surface of the steps facing the in-couplingedge surface makes an angle β with respect to a normal on a bisectingplane bisecting the light-emitting panel, wherein the bisecting planecomprises a bisecting line in the input edge surface, said bisectingline being parallel to the front wall and bisecting the input edgesurface, and wherein the angle β is at least 5°.

The bisecting plane makes an angle with the front wall of thelight-emitting panel and divides the second portion of thelight-emitting panel in half. The angle β is responsible for therotation of the angular intensity distribution of the returning lightsuch that the light on its way back is extracted over the entire lengthof the second portion of the light guide. Depending on the ratio of thesurface area S_(es) of the input edge surface and the largest surfacearea S_(tr) in the light-emitting panel, the form of the surface of thesteps can be chosen to be flat for ratios S_(tr)/S_(es)≦2.5. Otherwise,the angle β must be varied to ensure a widening of the angular intensitydistribution, and the form of the step surfaces will be preferablycurved. In the following, values for the average value of β, β_(av),will be discussed.

Preferably, the angle β_(av) is in the range from 5≦β≦25°. Two differentembodiments of the light guide ending and reflecting surface can bedistinguished. One embodiment includes an air gap between light guideending and (diffuse) reflecting surface. In this case, β_(av)>5° for alight guide material with refraction index n≈1.5 to ensure that lightextraction starts in the beginning of the second portion of the lightguide, where “beginning” means the side opposite to the input edgesurface of the panel. Equally, β_(av) must not be too large, to ensurethat light is still extracted close to the input edge surface.Simulations have shown that values for β_(av)>25° are disadvantageous.This can be attributed to the fact that the portion of light that afterreflection is outside the regime of total internal reflection increaseswith increasing values of β. Of course this also relates to the angulardistribution of the light when it is coupled into the light guide. Asmaller angular distribution at this stage makes larger values of βpossible and/or necessary, but to obtain this narrower distribution alarger thickness of the light guide at the input edge is required, whichagain is disadvantageous.

A preferred embodiment of the lighting system according to the inventionis characterized in that the surface of the steps facing the input edgesurface comprises a specular reflector on a side facing away from theinput edge surface. Such a specular reflector provides the desiredreflection of the light traveling from the input edge surface andstimulates the coupling out of light from the light-emitting panel.

In an alternative embodiment, the steps comprise a diffuser on a sidefacing away from the light-emitting panel, an air gap being maintainedbetween the steps and the diffuser or at least optical contact beingprevented between the light guide and the diffuse reflector. Preferably,the diffuser is made from a diffuse, highly reflecting material copyingthe shape of the steps of the first portion of the light-emitting panel.In this case the thickness of the air gap between light-emitting paneland the diffuser should be carefully controlled. Preferably, the diffusematerial has a very short penetration depth (<0.1 mm). The advantage ofemploying a diffuser is the very high robustness against light sourceradiation pattern and input geometries.

A preferred embodiment of the lighting system according to the inventionis characterized in that the ratio of the surface area S_(es) of theinput edge surface to the surface area S_(tr) in the light-emittingpanel at the transition between the first portion and the second portionof the light-emitting panel satisfies the relation 1<S_(tr)/S_(es)<10.Light originating from the light source and coupled into the secondportion of a light-emitting panel has an angular distribution whichvaries between approximately +42° and −42°, for typical light guidematerials with a refractive index of approximately 1.5. The lower limitfor the ratio of the cross section S_(tr) at the transition between thefirst and the second portion, where the light-emitting panel has thelargest thickness, and the surface area S_(es) of the input edgesurface, is given by the fact that the light-emitting panel iswedge-shaped in the second portion of the light-emitting panel. Theupper limit for the ratio S_(tr)/S_(es)<10 is determined by the wishthat the light-emitting panel should not become too thick. In principle,the dimensions (screen diameter) of, for example, the display devicedetermine the size (diameter) of the light-emitting panel. If the inputedge surface has a thickness of 2 mm, then the equation S_(tr)/S_(es)=10means that either the second edge surface or the cross-section of thelight-emitting panel parallel to the input edge surface at the locationof the largest thickness of the light-emitting panel will have athickness of 20 mm. The ease of manufacture of the light-emitting panelis also reduced in the case of such high ratios.

Preferably, the ratio S_(tr)/S_(es) satisfies the relation1.5<S_(tr)/S_(es)<4. Light-emitting panels with a S_(tr)/S_(es) ratio inthe preferred range can be readily manufactured in an (injection)molding process.

The light source used may be formed by LEDs, for example white LEDs ordifferent types of LEDs and/or LEDs of different colors which arecombined with one another. Colors may be mixed in a desired mannerthrough a suitable use of LEDs, for example for making white light of adesired color temperature. For this purpose, an embodiment of thelighting system according to the invention is characterized in that thelight source comprises one white LED or at least two light-emittingdiodes with different light emission wavelengths. Preferably, the lightsource comprises three light-emitting diodes. Employing a single whiteLED is advantageous because the widening portion of the light-emittingpanel is used for the homogenization of the light distribution in thelight-emitting panel. The LEDs preferably comprise the combinations ofred, green, and blue LEDs known per se, or, for example, combinations ofred, green, blue, and amber LEDs. LEDs with three light emissionwavelengths may also be realized by means of two LEDs with differentlight emission wavelengths, wherein the LEDs of one of the types are(partly) provided with a phosphor, such that the light emission of theLED is converted by the phosphor into light of a third, desired lightemission wavelength. A combination, known per se, of the red, green, andblue LEDs renders it possible to realize color changes independently ofthe status of the display device. The use of LEDs has the furtheradvantage that dynamic lighting possibilities are obtained. For thispurpose, a sensor is present at one of the edge surfaces for measuringthe optical properties of the light emitted by the light source duringoperation.

The quantity of light emitted by the LEDs is adjusted in that theluminous fluxes of the light-emitting diodes are varied. This control ofthe luminous flux usually takes place in an energy-efficient manner.Thus the LEDs can be dimmed without an appreciable loss in efficacy.Preferably, the intensity of the light emitted by the light-emittingdiodes is variable in response to the illumination level of a picture tobe displayed by the display device or in response to the level of theambient light. Preferably, the color point of a picture displayed by thedisplay device is determined by the lighting system. An (improved)dynamic range (for example contrast) of the picture to be displayed bythe display device is achieved thereby.

Preferably, each of the light-emitting diodes has a luminous flux of atleast 5 lm. Such high values are particularly useful forhigh-brightness, large-screen applications, i.e. with a value of thequotient of the light emitting panel luminous output and the paneldiagonal of more than 10 lm/inch. LEDs with such a relatively highoutput are also referred to as LED power packages. The use of thesehigh-efficiency, high-output LEDs has the specific advantage that thenumber of LEDs required for a desired, comparatively high light outputcan be comparatively small. This benefits the compact construction andthe efficiency of the lighting system to be manufactured. Furtheradvantages of the use of LEDs are a comparatively very long useful life,the comparatively low energy cost, and the low maintenance cost for alighting system with LEDs.

A considerable length is available for mixing the various light colorsuntil the desired color mixture has been reached, for example whitelight of a predetermined color temperature, in a lighting system with alight-emitting panel comprising a wedge-shaped portion widening from theinput edge surface, and in which the light cannot be coupled out duringits first travel through the widening (second) portion of thelight-emitting panel. Light-emitting panels of comparatively largedimensions can be realized in this manner with a light source whichcomprises in total, for example, six or even only three (high-output)light-emitting diodes with different light emission wavelengths. In analternative embodiment a single LED suffices. In the known lightingsystem, a light-mixing chamber of considerable dimensions is usuallynecessary for such a limited number of LEDs in order to achieve that thelight is sufficiently distributed and, in the case of multiple LEDs,mixed in the light-emitting panel so as to provide a uniform andhomogeneous coupling-out of light from the light-emitting panel in thedirection of the (picture) display device.

In a further preferred embodiment, the lighting system comprises controlelectronics for changing the luminous flux of the light source. Thedesired lighting effects are achieved by means of suitable controlelectronics, and the uniformity of the emitted light is improved. Whitelight is also obtained through a suitable combination of LEDs, for whichthe control electronics provide the possibility of adjusting the desiredcolor temperature.

A particularly compact lighting system is obtained through the measureaccording to the invention, with a high uniformity of the distributionof the light emitted by the lighting system. A more uniform illuminationof the display device is realized thereby in particular in the case of(picture) display devices.

The invention will now be explained in more detail with reference to anumber of embodiments and a drawing, in which:

FIG. 1A is a perspective view of a display device comprising anembodiment of the lighting system according to the invention;

FIG. 1B is a cross-sectional view of a detail of the lighting system asshown in FIG. 1A;

FIG. 2A shows the light emitted from a wedge-shaped light-emitting panelwith a single step which upon reflection at the surface of the step isoutside the regime of total internal reflection;

FIG. 2B shows the light emitted from a wedge-shaped light-emitting panelwhich upon reflection at the surface of the steps is outside the regimeof total internal reflection in a panel with a number of steps which arenot spaced apart, β taking alternately positive and negative values;

FIG. 2C shows the light which upon reflection at the surface of thesteps is outside the regime of total internal reflection andsubsequently is emitted from a wedge-shaped light-emitting panelaccording to the invention with a number of steps which are at a certaindistance with respect to each other, and

FIG. 3 shows the uniformity distribution of the light emitted by a frontwall of a light-emitting panel according to the invention with a singlestep at various values of the angle β.

The Figures are purely diagrammatic and not drawn true to scale. Somedimensions are particularly strongly exaggerated for reasons of clarity.Equivalent components have been given the same reference numerals in theFigures whenever possible.

FIG. 1A shows very schematically a perspective view of a display devicecomprising an embodiment of the lighting system according to theinvention. The lighting system comprises a light-emitting panel 1 of alight-transmitting material. The light-emitting panel 1 is manufactured,for example, from a synthetic resin, from acryl, from polycarbonate,from pmma, for example perspex, or from glass. Light is transportedthrough the light-emitting panel 1 during operation, utilizing totalinternal reflection (TIR). The light-emitting panel 1 has a front wall 2and a rear wall 3 opposite thereto. An input edge surface 4 is providedbetween the front wall 2 and the rear wall 3 of the light-emitting panel1. In the example of FIG. 1A, the input edge surface 4 is at an anglebetween 80 and 100° with respect to the front wall 2 of thelight-emitting panel 1. The input edge surface 4 is light-transmitting.The lighting system comprises a light source 6, 6′, . . . for example anumber of light-emitting diodes (LEDs). In the situation shown in FIG.1, light originating from the light source 6, 6′, . . . is incident onthe input edge surface 4 of the light-emitting panel 1 during operation,which light distributes itself in the light-emitting panel 1. Reflectormeans (not shown in FIG. 1A) may be provided between the light source 6,6′, . . . and the input edge surface 4 to guide the light of the e.g.LEDs into the light-emitting panel 1. In an alternative embodiment theLEDs are provided inside the light-emitting panel. To this end thelight-emitting panel may be provided with an indentation the shape ofwhich is substantially complementary to the shape of the light source.In this case the indentation functions as the input edge surface.According to the measure of the invention, the rear wall 3 in a firstportion 12 of the light-emitting panel 1 is provided with a multiplicityof steps 13, 13′, . . . (see also FIG. 113). In addition, a secondportion 22 of the light-emitting panel 1 widens from the input edgesurface 4.

Light is coupled into the light-emitting panel 1 at the thin end (inputedge surface 4) of second portion 22 of the light-emitting panel 1 andpropagates towards the first portion 12 of the light-emitting panel 1.In the example of FIG. 1A, light-emitting panel 1 reaches its largestcross section S_(tr) at the transition between the first and the secondportion. This transition is indicated in FIG. 1A by means of animaginary plane with reference numeral 122. In the first portion 12,light is reflected at surfaces 17 of the steps 13, 13′, . . . , saidsurfaces 17 facing the input edge surface 4. After reflection, the lightcan be coupled out of the light-emitting panel 1.

As a rule of thumb, the height h_(st) of a step is preferably in therange 0.1≦h_(st)≦0.5 mm; the last step, closest to the front wall, maybe chosen thicker (e.g. 2 mm) for mechanical reasons, the angle β ofthat particular step being reduced accordingly. In addition, thedistance d_(st) between two steps is in the range 0.1≦d_(st)≦10 mm.

The spatial frequency of the steps in the first portion of thelight-emitting panel can be chosen such that the brightness modulationfrom the many small bands of light can no longer be resolved, so thatthe brightness is averaged and appears homogeneous. Therefore, apreferred embodiment of the lighting system according to the inventionis characterized in that the number of steps is in the range from 25 to100.

A preferred embodiment of the lighting system according to the inventionis characterized in that the length l_(fp) of the first portion ascompared to the length l_(fw) of the front wall is in the range0.05≦l_(fp)/l_(fw)≦0.6. By selecting a ratio l_(fp)/l_(fw) in the givenrange, a further improvement of the uniformity distribution of the lightcoupled out of the light-emitting panel is achieved.

Preferably, the ratio of the surface area S_(es) of the input edgesurface 4 to the surface area S_(tr) in the light-emitting panel at theimaginary transition plane 122 between the first portion 12 and thesecond portion 22 of the light-emitting panel 1 satisfies the relation1<S_(tr)/S_(es)<10. Particularly preferred is a ratio S_(tr)/S_(es)satisfying the relation 1.5<S_(tr)/S_(es)<4. Light-emitting panels inwhich the S_(tr)/S_(es) ratio lies within the preferred range can bereadily manufactured in an (injection) molding process. A particularlysuitable ratio is S_(tr)/S_(es)≈2. For example, a suitable thickness ofthe input edge surface 4 is 3 mm, which means that for S_(tr)/S_(es)=2the largest thickness of the light-emitting panel 1 is 6 mm. Analternative suitable thickness for the input edge surface 4 is 1 mm,which means that the largest thickness of the light-emitting panel 1 is2 mm in the case of S_(tr)/S_(es)=2.

Since the second portion 22 of the light-emitting panel 1 widens fromthe input edge surface 4, light cannot leave the light-emitting panel 1during its first travel through the widening second portion 22. It isthus promoted that the light in its first travel through the secondportion 22 of the light-emitting panel 1 distributes itself and is mixedin the light-emitting panel 1. A good distribution and/or mixing oflight promotes the uniformity and the homogeneity of the light coupledout of the light-emitting panel 1. The second portion 22 oflight-emitting panel 1, as shown in FIG. 1A, acts as it were as alight-mixing chamber for light during its first travel through awidening light-emitting panel. According to the measure of theinvention, the light-mixing chamber is as it were integrated into thelight-emitting panel, which leads to a considerable saving in space. Aparticularly compact lighting system is obtained through the measureaccording to the invention, with a high uniformity of the distributionof the light emitted by the lighting system.

In FIG. 1A, the length l_(fp) of the first portion 12 is indicated aswell as the length l_(fw) of the front wall 2. Preferably, the ratio isin the range 0.05≦l_(fp)/l_(fw)≦0.6. If, by way of example, the lengthl_(fw) of the front wall 2 is approximately 300 mm, the length l_(fp) ofthe first portion 12 is approximately 30 mm, i.e. l_(fp)/l_(fw)≈0.1.Preferably, the number of steps 13, 13′, . . . lie in the range from 25to 100. In the example given (l_(fp)≈30 mm), a particularly advantageousnumber of steps is 50. Preferably, the height h_(st) of a step 13, 13′,. . . (see FIG. 1B) is in the range 0.1≦h_(st)≦0.5 mm. A light-emittingpanel 1 with such steps can be easily manufactured.

In the example of FIG. 1A, a diffuser is provided behind the steps 13,13′, . . . while an air gap 33 is maintained between the steps 13, 13′,. . . and the diffuser 32. In the example of FIG. 1A, the diffuser 32 isa block with a shape complementary to that of the steps 13, 13′, . . .of the first portion 12 of the light-emitting panel 1. Preferably, thediffuser 32 is made from a diffuse, highly reflecting material, forinstance Teflon, diffusely reflecting aluminum or a coating containinghighly reflecting particles such as halophosphates, calciumpyrophosphate, strontium pyrophosphate and titanium dioxide or e.g. anexpanded polytetrafluoroethylene coating. Such coatings, which show along penetration depth, can be used as a thin layer (0.1 mm) incombination with a specular reflector layer arranged therebehind toachieve higher reflectivity. Likewise, the white powders could beapplied onto the light guide, the approximately spherical grainsassuring that there is virtually no optical contact between powder layerand light guide, and a solid specular reflector positioned therebehindto keep the powder in place. Preferably, the diffuse material has a veryshort penetration depth (<0.1 mm). Likewise, the air gap has to be small(<0.1 mm) and controlled in thickness over the edge height (preferably±0.025 mm). In an alternative embodiment of the lighting system thesurface of the steps facing the input edge surface comprises a specularreflector on a side facing away from the input edge surface (not shownin FIG. 1A).

The light-emitting panel 1, during operation, emits light in thedirection of a display device for example a liquid crystal display (LCD)device 50 via a translucent diffuser 8 in order to further homogenizethe light issued from the light-emitting panel 1. The assembly of thelight source 6, 6′, . . . , the lightemitting panel 1, and the LCDdevice 50, whether or not accommodated in a housing (not shown in FIG.1A), forms a display device for displaying, for example, (video)images.The light-emitting panel 1 may further be provided with a sensor (notshown in FIG. 1A) for measuring the optical properties of the light.This sensor is coupled to control electronics (not shown in FIG. 1A) forsuitably adapting the luminous flux of the light source 6, 6′, . . . . Afeedback mechanism can be realized by means of the sensor and thecontrol electronics for influencing the quality and the quantity of thelight coupled out of the light-emitting panel.

In an alternative embodiment, the front wall 2 is provided with a lightredirecting foil.

Preferably, the light source 6, 6′, . . . comprises one white LED or atleast two light-emitting diodes with different light emissionwavelengths. Preferably, the light source comprises three light-emittingdiodes with a blue, a green, and a red light emission wavelength. Thesource brightness of a LED is usually many times higher than that of afluorescent tube. Furthermore, the coupling efficiency of light into thepanel with the use of LEDs is greater than with the use of fluorescenttubes. The use of LEDs as a light source has the advantage that the LEDsmay lie against panels made from synthetic resin. LEDs transmit hardlyany heat in the direction of the light-emitting panel 1, nor do theygenerate detrimental (UV) radiation. The use of LEDs, in addition, hasthe advantage that no means need be applied for coupling the lightoriginating from the LEDs into the panel. The LEDs in the lightingsystem may comprise suitably chosen clusters of blue, green, and redLEDs, or suitable alternative combinations of single-color or dual-colorLEDs, or a plurality of white LEDs with high luminous flux.

The LEDs used in a large-screen, high-brightness lighting system,characterized by a value of the quotient of the light emitting panelluminous output and the panel diagonal of more than 10 lm/inch, arepreferably LEDs which each have an optical power of at least 50 mW. LEDswith such a high output are also referred to as LED power packages.Examples of power LEDs are LEDs of the “Luxeon™” type (Lumileds), ofwhich the luminous flux per LED is 35 lm for red, 30 lm for green, 8 lmfor blue, and 40 lm for amber LEDs. In alternative embodiments, yellow,amber, cyan, magenta, and/or purple LEDs are used which have acomparatively high light output (whether or not with the aid of twospectral light emission wavelengths). It is also possible to use aplurality of white LEDs of high luminous flux. In further alternativeembodiments, red LEDs may be used in combination with blue LEDs whichare provided with a phosphor, such that the latter emit in two spectralbands, i.e. a blue and a green band.

Preferably, the LEDs are mounted on a (metal-core) printed circuit board(not shown in FIG. 1A). When power LEDs are provided on such a(metal-core) printed circuit board (PCB), the heat generated by the LEDscan be readily removed by the PCB through thermal conduction. Aninteresting embodiment of the lighting system is furthermore one inwhich the (metal-core) printed circuit board is in contact with thehousing of the display device via a thermally conducting connection.

FIG. 1B shows a cross-sectional view of a detail of the lighting systemas shown in FIG. 1A. In the first portion 12, a surface 17 of the steps13, 13′, . . . facing the input edge surface 4 makes an angle β≧5° withrespect to a normal 25 on a bisecting plane 20 bisecting thelight-emitting panel 1. The bisecting plane 20 makes an angle with thefront wall 2 of the light-emitting panel 1 and divides the secondportion 22 of the light-emitting panel 1 in half. To this end, thebisecting plane 20 comprises a bisecting line 21 in the input edgesurface 4, said bisecting line 21 being parallel to the front wall 2 andbisecting the input edge surface 4. Preferably, the angle β_(av) is inthe range 5≦β_(av)≦β25°. Particularly preferred is an angle β_(av) ofapproximately 15°–19°.

FIG. 2A shows very schematically the light in a wedge-shapedlight-emitting panel 101 with a front wall 102 and a rear wall 103 andwith a single step 113 (“single-facet back reflector”). It can be seenthat at the edge of the front wall 102 facing away from the light source(not shown in FIG. 2A) a bright band of light is formed. The brightnessof said band of light is substantially higher (factor of two or more)than the light emitted at the rest of the front wall 102 of thelight-emitting panel 101. A solution would be to exclude this brightband of light from the light contributing to illuminating the displaydevice. However, making this part of the front wall 102 not “available”is disadvantageous. Such a shielding is space consuming, which is aproblem if the light-emitting panel 101 is applied as backlight fordisplay devices, in particular for (laptop) LCD backlighting. Inaddition, the system efficiency would be reduced (depending on thegeometry this may be by more than 10%) which is particularly importantin the case of (laptop) display devices.

FIG. 2B shows very schematically the light emitted in a wedge-shapedlight-emitting panel 201 with a front wall 202 and a rear wall 203 andwith a number of steps 213 with no distance between the steps 213(“multi-facet back reflector”). The situation is improved with respectto the situation in FIG. 2A. Multiple bright bands of light are created.However, the bright bands of light occur still relatively close to theedge of the front wall 202 facing away from the light source (not shownin FIG. 2B).

FIG. 2C shows very schematically the light in a wedge-shapedlight-emitting panel 1 with a front wall 2 and a rear wall 3 and,according to the invention, with a number of steps 13, 13′, . . . spacedwith respect to each other (“stretched multi-facet back reflector”) (seealso FIGS. 1A and 1B). The bright bands of light are distributed over arelatively large part of the front wall 2 of the light-emitting panel 1.

FIG. 3 shows the uniformity distribution of the light emitted by a frontwall 2 of a light-emitting panel 1 according to the invention for aone-step facet at various values of the angle β (see FIGS. 1A and 1B).The length of the front wall 2 in this example is 250 mm. The ratio ofthe surface area S_(es) of the input edge surface 4 to the surface areaS_(tr) in the light-emitting panel at the imaginary transition plane 122between the first portion 12 and the second portion 22 of thelight-emitting panel 1 in the example of FIG. 3 is S_(tr)/S_(es)=2. Thelight source is located on the side of the light-emitting panel 1 wherex=0 mm. For values of the angle β≦10° it can be seen that too much lightis emitted at the beginning of the light-emitting panel 1 (i.e. at x≦100mm) resulting in a too low light level emitted by the remainder of thefront wall 2 (i.e. at x>100 mm). For values of the angle 22≦β≦15° it canbe seen that the light distribution is relatively flat, but relativelymuch light is emitted at the end of the light-emitting panel 1 (i.e. atx>225 mm). For values of the angle β of approximately 17°, the stretchedm# facet solution of this invention can spread the bright band at theend of the light emitting panel and combine that with a relatively flatlight distribution over the entire front wall.

It will be obvious that many modifications are possible to those skilledin the art within the scope of the invention. The scope of protection ofthe invention is not limited to the embodiments given. The inventionresides in each novel characteristic and each combination ofcharacteristics. Reference numerals in the claims do not limit the scopeof protection thereof. The use of the verb “comprise” and itsconjugations does not exclude the presence of elements other than thosespecified in the claims. The use of the indefinite article “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements.

1. A lighting system provided with a light-emitting panel (1) comprisinga front wall (2), a rear wall (3) situated opposite thereto, andfurthermore, between the front and the rear wall (2, 3), a translucentinput edge surface (4) for coupling light into the light-emitting panel(1), while at least a light source (6, 6′, . . . ) is associated withthe input edge surface (4), and while, in operation, light originatingfrom the light source (6, 6′, . . . ) is incident on the input edgesurface (4) and distributes itself in the light-emitting panel (1),characterized in that the rear wall (3) in a first portion (12) of thelight-emitting panel (1) is provided with a multiplicity of steps (13,13′, . . . ), and in that a second portion (22) of the light-emittingpanel (1) widens from the input edge surface (4) in a direction facingthe first portion (12) characterized in that a surface (17) of the steps(13, 13′, . . . ) facing the input edge surface (4) makes an averageangle β_(av) with respect to a normal (25) on a bisecting plane (20)bisecting the light-emitting panel (1), wherein the bisecting plane (20)comprises a bisecting line (21) in the input edge surface (4), saidbisecting line (21) being parallel to the front wall (2) and bisectingthe input edge surface (4), and wherein the average angle β_(av) is inthe range 5≦β_(av)≦25°.
 2. A lighting system as claimed in claim 1,characterized in that the surface (17) of the steps (13, 13′, . . . )facing the input edge surface (4) comprises a specular reflector (31) ona side facing away from the input edge surface (4).
 3. A lighting systemas claimed in claim 1, characterized in that the steps (13, 13′, . . . )comprise a diffuser (32) on a side facing away from the light-emittingpanel (1) while an air gap (33) is maintained between the steps (13,13′, . . . ) and the diffuser (32).
 4. A lighting system as claimed inclaim 1, characterized in that the height h_(st) of a step (13, 13′, . .. ) is in the range 0.1≦h_(st)≦0.5 mm.
 5. A lighting system as claimedin claim 1, characterized in that the distance d_(st) between two steps(13, 13′, . . . ) is in the range 0.1≦d_(st)≦10 mm.
 6. A lighting systemas claimed in claim 1, characterized in that the number of steps (13,13′, . . . ) is in the range from 25 to
 100. 7. A lighting system asclaimed in claim 1, characterized in that the length l_(fp) of the firstportion (12) as compared to the length l_(fw) of the front wall (2) isin the range 0.05≦l_(fp)/l_(fw)≦0.6.
 8. A lighting system as claimed inclaim 1, characterized in that the ratio of the surface area S_(es) ofthe input edge surface (4) to the surface area S_(tr) in thelightemitting panel (1) at the transition between the first portion (12)and the second portion (22) of the light-emitting panel (1) satisfiesthe relation 1<S_(tr)/S_(es)<10.
 9. A lighting system as claimed inclaim 8, characterized in that the ratio is 1.5<S_(tr)/S_(es)<4.
 10. Alighting system as claimed in claim 1, characterized in that the frontwall (2) is provided with a translucent diffuser (8).
 11. A lightingsystem as claimed in claim 1, characterized in that the front wall (2)is provided with a light redirecting foil.
 12. A lighting system asclaimed in claim 1, characterized in that the light source (6, 6′, . . .) comprises at least one white LED or at least two lightemitting diodeswith different light emission wavelengths.
 13. A lighting system asclaimed in claim 12, characterized in that each of the light-emittingdiodes has a luminous flux of at least 5 lm.
 14. A display deviceprovided with a lighting system as claimed in claim
 1. 15. A displaydevice as claimed in claim 14, which display device comprises a liquidcrystal display (50).